1. Field of the Invention
The invention pertains to the field of ship's steering and control and more particularly to a configurable digital autopilot for steering and controlling a marine vessel.
2. Description of the Prior Art
Ship maneuvering and course control have advanced from the tiller control of the rudder to the present day autopilots. Throughout the ages a bridge has been the command center of the ship and control functions have been performed at this location. Present day large ships, such as super tankers and container ships, may contain satellite control centers (remote stations) which provide better visibility of a maneuvering situation as, for example, in harbor navigation and ship docking. In such situations control of the ship may be passed to a remote station. In the prior art the remote stations, each of which may provide different operating modes, are hard wired to the steering system. Switching to a remote station is accomplished with multiple deck switches and numerous electromechanical relays.
Since automatic navigation is not always feasible and provision must be made for emergency situations, a steering system for a ship must provide, in addition to an autopilot, helm, and tiller steering modes of operation. In the prior art helm steering is provided with an analog helm. Typically, such steering systems utilize potentiometers to convert the angular rotation of the wheel to an analog voltage. This analog voltage requires an analog-to-digital conversion when digital signal processing is utilized in the autopilot to convert the helm signals to rudder orders. Further, such analog helms require mechanical and electrical calibration to scale the voltage to degrees of wheel rotation and to set the zero point. An analog helm has an absolute rudder order scale limited to a particular hard over rudder order and is not configurable for larger hard over values. This necessitates a mechanical stop on the potentiometer to limit its motion to the hard over rudder order on the scale. Traditionally the wheel of an analog helm traverses multiple turns between hard over left and hard over right orders to provide a desired fine rudder adjustment, thus requiring a gearing system between the wheel and the potentiometer to convert the wheel motion to potentiometer motion between the potentiometer stops. Each angular position of the wheel in an analog helm is associated with a predetermined rudder order. Therefore, to provide a bumpless transfer from a previous mode to an analog helm mode requires that the wheel be positioned to the angular rotation associated with the rudder order for the rudder position of the previous mode. If the wheel is not so positioned the rudder will move to the rudder angle associated with the wheel position, which in many instances causes the ship to experience a significant sudden rudder change or "bump".
Tiller control in the prior art, also known as non-follow-up control (NFU) for marine steering systems typically include one of three NFU operational modes; an NFU mode wherein the ship is tiller controlled, a jog NFU mode wherein tiller operation is temporarily invoked for a quick maneuver and then returns to the previous mode of operation, and force NFU which is an emergency takeover for tiller operation which remains in force until a new mode of operation is selected by the Operator. Only one NFU mode is provided and no choices are available. Further, the prior art NFU controls are not coordinated with follow-up (helm) mode solenoid controls. When invoked, the NFU overrides the pump solenoids from being actively driven by the autopilot, causing the autopilot to develop an increasing rudder error which causes all solenoids to energize at once as soon as the NFU device is de-activated. This pattern of use causes unnecessary hydraulic shock due to large changes in fluid flow through the pump system.
When a turn command is received in automatic modes, the autopilot controls the rate of turn in accordance with a selected turn rate. Conventionally, rate control turns are provided by rate pilots separate from the autopilot's maneuver and heading keeping control. These rate pilots utilize data from a rate gyro and the selected rate order to control the rudder. The rudder remains under the rate pilot control until it is deactivated, it does not stop at a desired course. The system Operator must determine when the desired course has been achieved and then transfer from the rate controller mode to the heading keeping mode or to helm steering. These prior art systems cannot provide for calibration of the rate pilot to the ship's turning characteristics. Additionally, the rate control does not accommodate changes in speed and ship load conditions. One autopilot of the prior art integrates rate control with other automatic controllers. This autopilot serves primarily large slow turning vessels and can accommodate rate orders to an upper limit of 60.degree./min. Control rates ramp up and down at a rate of zero to 60.degree./min. in twice the rise time (roll period of the ship). Rudder order update once per second is sufficient, and heading rate is filtered with a fixed time constant of 1/2 second. Speed scaling and calibration of proportional gain, integral time constant, feed forward gain, and rise time for two load conditions is provided with a range of values appropriate for large ships 20,000 DWT and up. Rudder order for rate control is not filtered prior to the output, smoothing is provided by the rudder control elements. This rate controller can be turned off prior to course changes to use the PID controller, but cannot be turned off during a maneuver.
Heading repeater and course selectors in prior art marine autopilots utilize step data from a gyro compass coupled to a compass repeater having a synchro or potentiometer that is manually positioned with respect to the repeater dial for heading selection to produce a proportional rudder control signal. Provisions for coupling to other compass signals such as synchro data or serial heading are generally not provided. The compass repeater must be synchronized with the gyrocompass whenever gyrocompass power is applied. Further, if the autopilot heading source is changed as, for example, from the primary gyrocompass to the backup gyrocompass, the repeater must be resynchronized to the new gyrocompass. Additionally, a prior art course selector is generally coupled to a potentiometer which produces a heading error voltage that is translated to a rudder order by the autopilot when in the automatic steering mode. Consequently, transfers from helm or tiller modes are not "bumpless" unless the Operator turns the pointer back to the lubber line before selecting the autopilot mode. When rudder control changes suddenly from helm or tiller control to a non-zero heading error control a "bump" occurs in these prior art systems.
Off course alarms in the prior art marine autopilots compare the ship's heading to a heading reference which is set by the Operator. An alarm is sounded when the heading error established by this comparison exceeds a heading error tolerance also set by the Operator. These devices must be aligned to every course change. Consequently, in a turning maneuver, during which heading error tolerance is exceeded constantly, chronic false alarms occur until the ship comes about on the new course. In these systems the alarm duration is generally fixed and a set heading reference cannot be exchanged with the reference for the steering maneuver to prevent the occurrence of the false alarms.
The servo amplifiers in the prior art autopilot systems use both analog and logic circuitry to determine rudder error and convert this error to a multistage solenoid control. The pump solenoids of the hydraulic system, which provide the rudder control, are wired to specific servo amplifiers and are energized and deenergized at values which are set when the system is installed, regardless of the autopilot's mode of operation or pump system selected in steering systems that contain more than one pump system. Staging pump solenoid operation follows a fixed sequence, and altering the staging to provide more efficient operation under certain system conditions is not available.